Filter operation on StringType after groupBy PERSISTED brings no results

ONLY HAPPENS WHEN PERSIST() IS CALLED

val data = sqlContext.read.parquet("/var/data/Saif/ppnr_pqt")
data.groupBy("vintages").count.select("vintages").filter("vintages = '2007-01-01'").first
>>> res9: org.apache.spark.sql.Row = [2007-01-01]

data.groupBy("vintages").count.persist.select("vintages").filter("vintages = '2007-01-01'").first
>>> Exception on empty iterator stuff

This does not happen if using another type of field, eg IntType
data.groupBy("yyyymm").count.persist.select("yyyymm").filter("yyyymm = 200805").first >>> res13: org.apache.spark.sql.Row = [200805]

NOTE: I have no idea whether I used KRYO serializer when stored this parquet.
NOTE2: If setting the persist after the filtering, it works fine. But this is not a good enough workaround since any filter operation afterwards will break results.
NOTE3: I have reproduced the issue with several different datasets.
NOTE4: The only real workaround is to store the groupBy dataframe in database and reload it as a new dataframe.

Query to raw-data works fine:

data.select("vintages").filter("vintages = '2007-01-01'").first >>> res4: org.apache.spark.sql.Row = [2007-01-01]

Originally, the issue happened with a larger aggregation operation, the result was that data was inconsistent bringing different results every call.

Reducing the operation step by step, I got into this issue.
In any case, the original operation was:

val data = sqlContext.read.parquet("/var/Saif/data_pqt")

val res = data.groupBy("product", "band", "age", "vint", "mb", "yyyymm").agg(count($"account_id").as("N"), sum($"balance").as("balance_eom"), sum($"balancec").as("balance_eoc"), sum($"spend").as("spend"), sum($"payment").as("payment"), sum($"feoc").as("feoc"), sum($"cfintbal").as("cfintbal"), count($"newacct" === 1).as("newacct")).persist()

val z = res.select("vint", "yyyymm").filter("vint = '2007-01-01'").select("yyyymm").distinct.collect

z.length

>>> res0: Int = 102

res.unpersist()

val z = res.select("vint", "yyyymm").filter("vint = '2007-01-01'").select("yyyymm").distinct.collect

z.length

>>> res1: Int = 103